1. Field of the Invention
The present invention relates to a circuit and method for switching pulse frequency modulation (PFM) and pulse width modulation (PWM), and more particularly, to a circuit and method for smoothly switching PFM and PWM based on the status of output loading.
2. Description of the Related Art
FIG. 1 shows a prior art control circuit of a power transistor 11, which is used in a photoflash capacitor charger or a backlight controller. For prior applications, usually output loading must be measured first, and thereafter, connecting the gate of the power transistor to either the PFM or PWM will be determined. For PFM signals, the more loading there is, the higher the frequency is. For PWM signals, the more loading there is, the wider the pulse is. Generally speaking, a heavy output loading is suitable to PWM, but a light output loading is suitable to PFM. The prior circuit in FIG. 1 detects current I of the inductor L and power transistor 11 to determine an output loading condition, i.e., the so-called current mode. Therefore, an analog IC which includes the switching circuit between PFM and PWM must incorporate the power transistor 11. The prior method has the disadvantage of a large chip area and the inability to choose the desired power transistor by users themselves, e.g., a power transistor 11 with small impedance, and therefore a lot of inconveniences occur.
Besides, the prior switching method between PFM and PWM usually adopts a skip mode or burst mode. No matter which mode is chosen, the switching process is not entirely smooth, so the output voltage will jitter. If such a prior circuit is used in backlight controlling, the output display quality is not very good.
In short, avoiding the manufacture of a power transistor 11 and a switching circuit in a single IC and smoothly switching between PFM and PWM signals is an important issue right now.